Electromechanical vibration-translating device such for example as loud-speakers



Dec. 30, 1930. M. TROUTON 1,786,584

ELECTED-MECHANICAL VIBRATION TRANSLATING DEVICE SUCH FOR EXAMPLE AS LOUD SPEAKERS Filed July 19, 1928 Patented Dec. 30, 1930 UNITED-STATES ranar -omen MAURICE 'rnou'rom orB cxmenAmsnm-E, ENGLAJD, Assmnon'ro LIMITED, or nonnomnnenann p ELECT- ROMECHANIGAL VIBRATION-reexamine amvren r LOUD-SPEAKERS w'mELEss were soon For: EXAMPLE'AS- Application filed July 19, 1928, Serial no. 293,961, and in (sea Britain Auguste, 1927. 4

The present invention is'concerned with improvements in or relatlng to electrical devices of the type in which one member is caused to move relatively to another member under the influence of electrically generated forces acting between adj acent sur-- faces on the respectivemembers orin which electrical oscillations are generated by the relative movement of two members, the move ment being at leastin part rotational.

I'f reliance is placed on a mechanical re- 7 electric currents into mechanical displacements, for example, the relation between current and displacement has not been linear. 3 The extent of the departure from linearity can bereduced by makingthe working displacements a small fraction of the normal of the instrumentis impaired.

In a companion application No. 257 ,971 for electrically operated vibratory devices, filed February 29, 1928, which also relates to armatures the vibrational movement of which is at least partially rotational, attention is called to theadvantages resulting from the surface of the armature being disposed, when no oscillatory currentis passing, at an angle the armatu1e,this disposition being effected either by an inclination of the surface of the reed-support, or by inclining the armature armature surface relatively to that of thepole pieces, for example, by mounting the magnet so that it was rotatable about an axis. a

One object of the present invention is to minimize the injurious effect ofreactiveforces acting upon an armature during viuse'the'se forces to advantage.

Another object of the present invention is to provide an electro-magnetic vibratory device which is eflicient and in which a substantially linearerelation between a plied curbration due to the loa'dwhich it drives, or to rent and resulting displacement o a point in the armature maybe obtained, Whether the pole pieces actuating the armature'areon the same or onopposite sides of thearmature.

' A further object of the invention is to tor in which equal and opposite currents fed to the'fdevice produce equal and opposite displacements of a point on the armature.

According to one feature of the present invention, in a devi'ceof the type set forth, the memberxto be driven by a vibratoryrarmature, or thememberadapted to drivethe armature, is connected to the armature at a point (referred to as th'e' point of attachment) situated between the centre and the heelthereof. For the purpose of this specification the armature isto be regarded as those parts of the vibratory member which lie, atany moment, vertically above the stationary co-operating surface (assuming that the stationary surface is disposed horizontally) and the end of the armature nearer the real or virtual'axisj of rotation thereof will be referredto as the heel of'the arma "provide an efiicient electro-magnetic vibradevice in whlch' the magnet poles are turefthe opposite end of the armature being referred. to as the toe. to the surface of the pole pieces whichldid not change in sign during the vibration of i According to another feature of the present invention, the armature is so dimensioned and disposed that as it moves from its position of closest proximity toacooperating surface away from said surface, the point at whichfithe resultant attractive force (referred to as the centre of force) acts, moves towards the toe of said armature. V

Accordlng to a further feature of the present lnvention, the parts are so dlmensioned and disposed that, asthe armature moves, the

centre of force moves in such a manner that the moment of the resultant attractive force outermost working position.

tially linear law with the displacement of the point of attachment (towards and away from the co-operating surface) from the position of rest. In the preferred arrangement the point. of attachment is so selected that the centre of force moves from one side to the other side of the point of attachmentas the armature moves from its innermost to its It is preferably arranged that the change inthe radius of rotation of the centre of force during movement of the armature shall be sufficiently large in relation to the radius itself, to provide compensation for relatively large changes of attractive force.

The invention is illustrated in the accompanying drawings in which Figs. 1 to 3 are views in side elevation plan and end elevation respectivelyof a part of an electromagnetic vibratory device in accordance with the invention, the View in Fig. 1 being a section on the line R-R of Fig. 2, and

Figs. & and 5 are diagrams illustrative of the invention.

Referring to Figs. 13, a magnet (coils of which are indicated at C) is arranged with its pole pieces 1 and 2 projecting inwards, a small air gap being left between the tips of the pole pieces and the magnetic circuit being closed excepting for this air gap. The upper surfaces of the pole pieces adjacent the air gap, which will be referred to as the pole surfaces, will be regarded as one of the surfaces above referred to, namely the fixed surface. An armature 3, the under surface of which acts as the other surface above referred to, namely the movable surface, is carried upon a cross piece 4 which is clamped at its two ends by means of screws passing through holes 5. The armature may therefore be regarded as mounted for rotational vibration about an axis 6 which is upon the centre line of the cross piece 4. The cross piece 4: is thus adapted to twist and furnish the required restoring force. The width of the armature is, as shown, greater than the width of the air gap. The under surface of the armature will at first be considered to remain flat and for simplicity, its plane will be assumedto contain the axis of rotation 6, since very little error is introduced by this latter assumption. The axis of rotation of the armature surface will thus be assumed to be at B (Fig. 1). The law of magnetic force between the two surfaces is a complex one, but it may be assumed with sufficient accuracy for many practical purposes that the total magnetic force is equal to where 0 is a constant, aim is an element of length along the armature, (that is in a direction normal to the axis B), y is the distance between the element (Zn and the pole surface measured along its arc of rotation, p is the width of the magnet poles, the distance a: being measured from the edge of the poles nearer the axis of rotation, and A is a constant. It is interesting to note that the constant A may be regarded as the length of air gap (having the same cross section as the working air gap between armature and poles) which has the same reluctance as the iron path. A will be referred to as the equivalent minimum air gap.

In order to ascertain the distribution of magnetic force for any position of the armature Within working limits, it is convenient to determine the point in the armature about k=,m= in= 6 the distance,

in a vertical plane through the axis of rotation of the armature, between the co-operating surfaces upon the armature and the magnet poles 8 being measured positive when the two planes intersect between the axis of rotation of the armature and the co-operating surfaces, 6 is the value of 3 (that is to say the length of air gap) at the heel of the armature, a, is the distance of the centre of force from the heel of the armature and r is the distance of the axis of rotation from the nearer edge of the poles. The resultant magnetic force may be regarded as acting at this point distant a from the heel of the armature.

From Equation (2) the interesting fact may be shown that if is 1 the centre of force does not move during displacement of the armature. In other words, if the axis of rotation of the armature (considered of course to lie in the plane of the armature surface) is situated below the plane of tie magnet poles by an amount equal to the equivalent minimum air gap, the centre of force remains a fixed point in the armature during movement of the armature.

For under these conditions we have:

the toe of the armature as the armature moves away from the magnet poles.

matically an imaginary case in which 8 A' and in which the armature3 is constrained In: Fig. at there is illustrated diagramto execute parallel motion, that is to say, 1' is .made infinite, thearmature being shown in two positions, the full'line postion being that of contact-at one end. with the pole Qand the dotted line position beingfurther from the pole; The-position of the centre of force in the two armature positions is indicated by the'arrows.

-l-It' has been found that, when a vibratory device of the type shown in Figs. 1 to 3 is used to .drive the conical diaphragm of a loud speaker through the rod 7 there is one position of the point of attachment of the diaphragm to the armature which gives better results than any other, particularly in regard to the reproduction ofthe lower tones, and this position has always beenyfound to lie between the centre and theiheel of the armature. The best'point is usually found to beat adistance from the heel of the-armature between and%- of the length of the armature, or in other words between onesixth and one-tenth of thelength of the arma ture back from the centre thereof, measured in a. direction normal to the axis of rotation thereof. It has also been found that this optimum position approximately coincides with the position of the centre of force in the position of rest ofthe armature, and when the armature moves from its innermost to its:

outermost p ositiondduring vibration, the centre of force moves from one side of the point of attachment to the: other sidethereof.

A possible explanation of this will be given later.

.Tliemovement of the centre of magnetic force towards the axis of rotation of the armature'as thearniature approaches the poles indicated Equation .(2) :is utilized, according to a feature of the present invention, to assist in providinga linear relation between the current passed through coils surrounding the magnet poles and the displacement of the point of attachment produced by the current while maintaining efiiciency by allowing the armature in its innermostposition at maximum amplitude to all but make It is known that the magnetic force on an armature increases too rapidly at small air gaps as compared with the change at larger air gaps to give such a linear relation if a restoring force obeying Hooks law is used and if the distribution of the magnetic force in the armature remains constant. Moreover, for simplicity and cheapness it is highly desirable to use a restoring force of this nature than if it is large and it has been found that for most purposes it is desirable that caused to mb v'e towards the axis of rotation of i i .stantially constant for all equal displace nients of the-armature within its working range. a I i Itwillbe clear that any given movement of the centreof force will have a greatereffect if the radius of rotation of the armature is short in relation to the length of the armathe length of the armature should beat least equal to the distance between the heel of the armature and theaxisof rotation. i I

It is emphasized here that Equation (2) is'only true if the armature is of such stiffness that its flexure can be disregarded. With such an armature it is found that in most practicalcases the value of 8 has to be made ratherhigh inorder to get a suflicient movement of the centre of force to give a sufficiently close approach tolinearity,-and this leads to lack of sensitivity. It has however, been discovered that if the armature be made so 7 that its flexure due to the forces acting upon it is not negligible, this flexure can be made to assist in securing linearity. 'No rule fordetermining the "optiumum flexibility can be given but,'with the assistance of the probable explanations of the manner in which the flexibility affectsthe performancegivm below, the best value in any particularcase can readilybe found experimentally. Dimensions of one working'arrangement are also given. i

It can be shown that when the air gap is small in relation to A so that the square of the v and a relatively slow movement of the'centre of force is required,the rate of movement preferably tending towards zero at contact,

whereas at larger air gaps itis of advantage to increase the movement of the centre of force over what is normally given by a plain rotational movement. Flexing of the armature tends to fulfil both these desiderata.

In estimating the effect of the magnetic force in producing flexure we may regard the armature as a beam anchored at the heel of the armature. This will of course only be the case if, as in the device shown in Figs. L3, the armature is considerably more flexible 'than the member between it and the cross bar. l

Thusthe arm of the moment of the resultant magnetic force is shorter in this case than when rotation about the axis of rotation of the armature is being considered and a small movement of the centre of force will consequently have a greater effect. In both the innermost and outermost armature positions, flexure tends to reduce the toe air gap, that is to say, the effect in both cases is to make .the armature move'lnore nearly parallel to itself than it would if it did not bend. If one surface moves parallel to itself and parallel to a co-operating surface, the centre of force does not move. If one surface moves parallel to itself but at an angle to a co-operating surface, the movement of the centre of force is greater than it would be with pure rotational movement. Thus it would appear that the fiexure tends to reduce the movement of the centre of force towards the heel when the armature approaches the magnet pole and to increase the movement of the centre of force to wards the toe when the armature moves outwards. If the arrangement is such that the amount of fiexure is greater, owing to the movement of the centre of force (which may be regarded as acting at a short arm) in the outermost armature position than in the innermost position, the effect of flexure will be, as stated, to assist in giving the linear relation desired. i

Fig. 5 is a diagram illustrating the foregoing and also offering a possible explanation of the advantage of connecting a diaphragm to the armature at a point between the centre and the heel of the armature. The armature 3 is shown in full lines in its outermost position during vibration and in dotted lines in its innermost position. The armature is flexible and the disposition and direction of the forces acting upon it in the two positions are indicated by the arrows. M and M are the resultant magnetic forces in the two positions respectively, L and L are the forces at the point of attachment due to the mass of the diaphragm or-other driven load and R and R are the mechanical restoring forces. It will be seen that there is a tendency for the system of forces apart from the dynamic forces to produce flexure of the armature in such a manner that the point at which the force M acts is nearer the toe of the armature than if the flexing did not take place. The force M will also act at a point which is further from the heel than if the armature remained straight but, as already stated the flexure may be made greater in the outermost ditions as to the desired closeness of approach to linearity.

If the centre of force moves as shown from one side to the other side of the point of attachment during movement of the armature, it can be shown (for any given position of the heel of the armature and taking into account the fact that for equilibrium the moments of the forces, L and M about B, in the dynamic case, must equal the moment of the force M alone about B, in the static case) that the inertia forces L and L both tend to increase the flexing effects which take place statically and which have been described above.

For the ratio of the flexing moment when the inertia force is acting to that when the inertia force is not acting is equal to L rq for the full line position of Fig. 5, where p and g are the ratios of the distances of the forces M, and L respectively from the axis B to the distance of the force R, from that axis.

The equivalent to expression (4) for the dotted line position of Fig. 5 is 2 1+ where p and 9 have the same significance in relation to forces L and M as they had in expression (4) in'relation to L, and M It will be seen from these two expressions that if p g 1 in expression (4) and if g p 1 in an expression (5), the flexure is increased by the inertia force in both positions of the armature.

.Moreover it appears desirable from these expressions that L and L should attain zero value when p becomes equal to g in order that the arrangement should be symmetrical and to prevent the introduction of harmonics, and this is apparently equivalent to stating that the load should be attached to the armature at a point which is coincident with the centre of force in the position of rest of the armature.

It should be pointed out that the amount of flexure which has been found desirable in practice is extremel small, for example of the order of 0.01. m/m, but since the total amplitude of vibration of the point of attach- .ment may not exceed 0.05 m/m and the final air gap will be a small fraction of the amplitude, it will be clear that a small amount of flexure may have a very important effect.

In order to decrease the size of the minimum air gap where a flexible armature is used, it may sometimes be desirable to shape one or both of the co-operating surfaces in such a manner that a closer approach to uni- .form contact over the whole of the surfaces is obtained after the armature has flexed than is possible if both surfaces are-flat when the poles it is'convenient to adjust theheight of the platforms upon which the crossbar is v clampec, for example by the insertion or removal of packing washersplaced between the cross bar and the platform in the'neighbourhood of the holes 5. After each adjustment the relation between applied Current and displacement of the point of attachmentis determined, for example with the aid of a Kelvin optical lever, and this is continued until the desiredrelation is obtainedr The final test will, however, in a vibratory device be a dynamic one and for this purpose a number of different pure frequency electrical oscillations may be 'fedsuccessively to thedevice and either a record may be made of the wave form of the mechanical oscillations produced (for example by optical means) or the percentageof harmonic frequencies'produced may be measured with the aid of suitable filters. V v

In one loud speaker movement embodying the present invention, th'e magnet pole surfaces are mounted for rotary adjustment about an axis in their plane and in the same vertical plane as the point B.

If the device is intendedito work over a considerable range of maximum amplitudes it has been found advantageous however, to

make the radius of rotation of the magnet pole surfaces less than above stated in order to secure greater uniformity of performance at different settings of the adjusting means.

In one case the radius of rotation of the magnet poles was reduced by 2 m/m with good results. The cross bar is mounted upon a platform the upper surface of which is 0.02 m/m below the axis of rotation of the magnet poles. The length of the cross bar between the end supports is 38 m/m, the depth of the cross bar is 5 m/m and its width 1.2 m/m. The radius'of rotation of the heel of the armature is 6.0 m/m, the length of the armature is 9.6 m/m, the thickness of the armtaure 1.7 m/m and its width 6 m/m. The magnet is a U-shapedcobalt steel magnet (86% cobalt) 1 15 m/min length along the centre line, including the pole pieces, and having a cross sectional area of 85 sq. m/m. The air gap between the pole pieces is 2.3 m/m. The point of attachment is 1.2 m/m back from the centre of the armature.

Although the invention has been described as applied to an armature'with one particular type of restoring force, it is not so limit- .ed. A pivoted armature with a separate recorrect relaabout a-virtual-axis as described in my prior application No. 257,971.

Iclaimzv 1. A vibratory device comprising an arma- I turemounted for vibration about an axis,.a magnet pole surface adapted-to co-operate with a surface of said armature, means adaptedto vary theforce between the armature and magnet'pole surface inaccordance with electricaloscillations, a member adapted tobe driven bysaid armature, and'means connecting the armaturewith said member, said connecting means being attached to the armature at a pointsituated between the centre of said cooperating surfaces and the ends thereof nearer said axis.

said surfaces and between said centre and the axis. 7 V I 4. A device for the inter-conversion of electrical and mechanical energy comprising an armature mounted for vibration about an axis, a surface'adapted to cooperate-with said armature, means for providing an attractive force between the armature and said surface, restoring means for the armature, and means, adapted to transmit mechanical vibratory energy, attached to the armature at a point within the magnetically active part of the armature, whereby the centre of attractive force will move from one side to the other side of said point as the armature moves, during vibration, from its position of closest proximity to said surface to' its position furthest fromsaid surface.

5. A'Jdevice for the inter-conversion of an armature mounted for vibration about an 1 2A device for the inter-conversion of electrical and mechanical energy comprising axis,'a surface-adapted'to' cooperate with the armature, and means for providing an at-.

tractive force between the armature and said surface, the'side of .said armature adjacent said surface being of convex form when the armature is unstressed. Y I I 6. A device (for the inter-conversion of electrical and mechanical energy-comprising an armature mounted for vibration about an axis, a convex surface adapted to cooperate with said armature, and means for providing an attractive force between said armature and said surface. q a Y 7. A vibratory device comprisingan armature mounted for vibration about an axis, a magnet pole surface adapted to co-operate with a surface of the armature, means adapted to vary the force between said cooperating surfaces in accordance withelectrical oscillations, a member adapted to be driven by the armature, and means connecting the armature with said member, said means being connected to the armature at a point situated between the centre of said cooperating surfaces andthe ends of said surfaces near said axis, and the radial length of said cooperating surfaces being at least of the radius of rotary vibration of the centre of said cooperating surfaces. I

8. A device for the inter-conversion of electrical and mechanical energy comprising an armature mounted for vibration about an axis, a magnet pole surface adapted to cooperate with a surface of the armature, and means, adapted to transmit mechanical vibratory energy, connected to the armature at a point within the magnetically active part thereof.

9. A device for the inter-conversion of electrical and mechanical energy comprising anarmature mounted for vibration about an axis, a magnet pole surface adapted to cooperate with a surface of the armature, means for providing an attractive force between the armature and said surface, and restoring means for said armature, the radial length of the said cooperating surfaces being at least of the radius of rotary vibration of the lengthwise centre of said surfaces and the disposition of the armature being such that the centre of attractive force thereon will move away from said axis when the armature moves during vibration from its position of closest proximity to said surface to its position furthest from said surface.

10. Adevice for the inter-conversion of electrical and mechanical energy comprising an armature mounted. for vibration about an axis, a magnet pole surface adapted to co operate with a surface of the armature, means for providing an attractive force between the cooperating surfaces, and restoring means for said armature, the radial. length of said cooperating surfaces being at least of the radius of rotary vibration of the lengthwise centre of said cooperating surfaces and the disposition of the armature being such that the point in the armature at which the resultant attractive force upon the armature acts, moves so that the moment of said resultant force above said axis varies according to a substantially linear law with the displace-- ment of a point on the armature towards and away from said surface.

11. A vibratory device comprising an armature mounted forvibration about an axis, a magnet pole surface adapted to cooperate with a surface of the armature, means adapted to vary the force between said cooperating surfaces in accordance with electrical oscillations, a member adapted to be driven by the armature, and means connecting said member to the armature at a point situated in the magnetically active part of the armature between the centre of said cooperating surfaces and the end of said surfaces nearer said axis, the portion of the armature which co-operates with said pole surface being flexible in relation to the portion thereof between said co-operating surfaces and axis.

12. A device for the inter-conversion of electrical and mechanical energy comprising an armature mounted for vibration about an axis, a magnet pole surface adapted to cooperate with a surface of the armature, and means, adapted to transmit mechanical vibratory energy, connected to the armature at a point situated between the centre of said co-operating surfaces and the ends of said surfaces nearer said axis, the portion of the armature which co-operates with said pole surface being flexible in relation to the porion thereof between said co-operating surfaces and said axis.

13. A device for the inter-conversion of electrical and mechanical energy comprising an armature mounted for vibration about an axis, a magnet pole surface adapted to cooperate with a surface of the armature, means for providing an attractive force between said co-operating surfaces, restoring means for the armature, and means, adapted to transmit mechanical vibratory energy, at tached to the armature at a point within the magnetically active portion of the armature. whereby the centre of attractive force is adapted'to move from one side to the other side of said point when the armature moves, during vibration, from its position of closest proximity to said surface to its position furthest from said surface, the portion of said armature which co-operates with said pole surface being flexible in relation to the portion thereof between said co-operating surfaces and axis.

In testimony whereof I have signed my name to this specification.

MAURICE TROUT N. 

